1. Field of the Invention
The invention pertains to a method for metering an ammonia-releasing reducing agent into an exhaust gas stream of an internal combustion engine operating with excess air in an exhaust gas post-treatment system.
2. Description of the Related Art
In addition to solid particles, nitrogen oxides are restricted exhaust gas components that form during combustion processes. The permitted levels of these components is continually being lowered. Various methods are in use today to minimize these exhaust gas components in internal combustion engines operated in motor vehicles. Lowering the levels of nitrogen oxides is usually done with catalysts. To raise the selectivity and the NOx conversion rates, a reducing agent is present in the oxygen-rich exhaust gas.
These approaches have become known under the collective term “SCR method”, where SCR stands for “selective catalytic reduction”. They have been used for many years in the power plant industry and recently also in internal combustion engines. A detailed description of these methods can be found, for example, in DE 34 28 232 A1. V2O5-containing mixed oxides such as those in the form of V2O5/WO3/TiO2 can be used as SCR catalysts. The amounts of V2O5 present are typically in the range of 0.2-3%. The use of iron-containing and/or copper-containing zeolites is also conceivable.
Ammonia or compounds which split off ammonia such as urea or ammonium formate in solid form or in the form of a solution are used as reducing agents in practical applications.
Urea decomposes at high temperatures into isocyanic acid and ammonia:(NH2)2CONH3+HNCO  (1)
The isocyanic acid is hydrolyzed by water in the exhaust gas to NH3 and CO2:HNCO+H2ONH3+CO2  (2)
Upon complete hydrolysis of one mole of urea, therefore, two moles of ammonia and one mole of carbon dioxide are formed:(NH2)2CO+H2O2NH3+CO2  (3)
As a result, the hydrolysis of urea provides the same reliable reducing agent as that used in the power plant industry, namely, ammonia.
One mole of ammonia is required to convert one mole of nitrogen monoxide:4NO+4NH3+O24N2+6H2O  (4)
The ratio of NH3 to NOx is called the feed ratio α.α=NH3/NOx 
In the case of an ideal catalyst, this means that, at a feed ratio of one, all of the nitrogen oxides are reduced; that is, a NOx conversion of 100% is achieved, because the following is true for the NOx conversion XNOx:XNOx=(cNOx,0−cNOx)/CNOx,0 where
CNOx,0: raw NOx emissions, ppm, and
CNOx: NOx emissions after the catalyst, ppm.
If, to form NO2, a platinum-containing NO oxidation catalyst is installed upstream of the SCR catalyst:2NO+O22NO2  (5)then the SCR reaction can be greatly accelerated, and the low-temperature activity can be significantly increased.NO+2NH3+NO22N2+3H2O  (6)
Nevertheless, in the presence of NO2, it must also be expected that the emissions of nitrous oxide will also increase according to the following reaction:2NH3+2NO2+½O22N2O+3H2O  (7)
In the case of internal combustion engines operating in motor vehicles it is difficult to use the SCR method to lower the nitrogen oxides, because the operating conditions are always changing. For example, the exhaust gas temperatures, the quantities of exhaust gas, and the raw NOx emissions are subject to frequent fluctuations. This makes it difficult to add the proper quantities of the reducing agent. On the one hand, the goal is to achieve the highest possible conversion of nitrogen oxides, but at the same time care must be taken not to allow the emission of nitrous oxide, isocyanic acid, or unconsumed ammonia.
To meter the reducing agent for the SCR method in motor vehicles, there are currently two different ways in which the correct metered quantity of reducing agent is determined.
The first is a pure open-loop control method without any feedback for determining the actual emissions downstream of the catalyst system. The metered quantity is determined in this case with the help of models based on data acquired and/or stored in the memory of an electronic engine control device of the internal combustion engine in the form of tables, curves, characteristic diagrams, or functions and possibly with the help of sensors for determining the catalyst temperature and the quantities of NOx and exhaust gas. The raw emissions of the engine are calculated, for example, from the injected quantity, the engine rpm's, the injection pressure, and fuel/air ratio, etc. The possible NOx conversions and the metered quantities of reducing agent required to achieve them depend in turn on the catalyst temperature, on the raw NOx emissions, on the quantity of exhaust gas, etc. The actual emissions downstream of the system are not detected and thus have no effect on the metered quantity (DE 43 15 278 A1, DE 195 36 571 A1, DE 199 06 344 A1, EP 898 061 A1).
The disadvantage of this method is that, because of the absence of feedback concerning the actual emissions, it is almost impossible to compensate for errors, defects, or environmental influences.
The second possibility is a standard closed-loop control circuit with HNCO, N2O, NOx, and/or NH3 sensors downstream of the system. For this purpose, the actual values currently being supplied by the sensors are compared with the nominal values, and the metered quantity is adjusted continuously. Nevertheless, the problem of permanent closed-loop control consists in the inertia of the system and of the sensors and simultaneously in the highly dynamic way in which an internal combustion engine operates in a motor vehicle. For example, during an acceleration process or an increase in the load on an exhaust gas-turbocharged internal combustion engine, the NOx emissions can rise by a factor of 10 within one second. In the case of naturally aspirated engines, the rise occurs even faster because of the absence of inertia of the exhaust gas turbocharger. The same is also true when loads are shed or on the transition to operation in push mode.
The sensors used to determine the emissions are not able to detect these highly dynamic processes. One of the reasons for this is the inertia of the sensors. The typical t90 time, that is, the time at which 90% of the end value is reached, of these sensors is found in the range of 300-500 ms. Another reason is the necessity to position the sensors behind the catalyst system. Thus the gas transit time from the discharge point from the cylinder head to the discharge point from the catalyst system alone is in the range of 200-2,000 ms, depending on the volume flow rate of the exhaust gas and the volume of the exhaust gas system.
To remedy this problem, an exhaust gas post-treatment system and a method for controlling this exhaust gas post-treatment system of the general type in question, especially for an internal combustion engine, is proposed in DE 101 00 420 A1, which partially solves the problem. A predetermined quantity of reducing agent is supplied to the exhaust gas post-treatment system, which comprises at least one catalyst, as a function of the operating state of the internal combustion engine and/or of the exhaust gas post-treatment system. The quantity of supplied reducing agent is adapted. For this purpose, the discharge of nitrogen oxides or of ammonia is measured by the use of sensors while the internal combustion engine is operating under steady-state conditions, and these measurements are compared with nominal values which have been stored for this steady-state operating condition. If a deviation is found, the control unit of the exhaust gas post-treatment system determines a correction value, by which the supplied quantity of reducing agent is then adjusted, i.e., adapted.
The disadvantage of this system is that, because of the previously described inertia of the system, the adaptation can occur only during a relatively long period of steady-state operating conditions under the given operating parameters of the internal combustion engine. The phase “steady-state operating conditions” means that the operating variables, which determine the metered addition of the reducing agent, may not change or may change to only a minimal extent. Such steady-state operating conditions do not occur for long periods of time during certain types of operation of the internal combustion engine, e.g., when it is operated in a vehicle driven in city traffic. As a result, the quantity of reducing agent cannot be corrected for long periods of time, and therefore increased amounts of pollutants such as nitrogen oxides or ammonia are discharged from the vehicle.
Another approach to solving the problem described above can be found in DE 195 36 571 A1. Here a method and an associated device for metering the input quantity of a reducing agent into the exhaust gas or exhaust air stream of internal combustion systems, especially of internal combustion engines, with a downstream catalyst are described. The setting of the input quantity of the reducing agent is accomplished, based on operation-relevant parameters of the internal combustion system, of the exhaust gas, and of the catalyst by way of characteristic curves (or diagrams), wherein the position of the characteristic curves (or diagrams) is inspected and adjusted to the actual state and to the actual operating conditions of the internal combustion engine, of the exhaust gas, and of the catalyst. What happens therefore, is that the characteristic curves or characteristic diagrams are adapted by comparison of the actual pollutant concentration determined by the sensors with the stored nominal values.
When this approach is used, the considerable inertia of the system, especially of the sensors, again means that an inspection can be conducted only when the internal combustion engine is operating under steady-state conditions, and therefore that the disadvantages described above with respect to DE 101 00 420 A1 are still present here.